1. Field of the Invention
The present invention relates to a interlocking device and a packaging bag therewith wherein the interlocking device has sufficient heat stability against retort sterilization treatment and dimensional change after said treatment small enough not to be impaired its reclosability.
2. Background of the Invention
A packaging bag which is made reclosable by attaching a interlocking device having a pair of male and female profiles at its opening (zippered bag) are widely used in a lot of fields such as food, medicine, and miscellaneous goods etc. Various methods are proposed as a process for the manufacture of the bag with an interlocking device. For example, a method to extrude a tubular film combined a pair of male and female profile on it with circular die, a method to attach the interlocking device extruding on to a film, a method to attach an interlocking device prepared beforehand on to a film substrate by heat sealing or setting with adhesives.
However, since the first and second methods have drawbacks such as the limitation of resin types applicable, difficulty to adopt them to multi-layered film substrate, limitation to deploy to various sizes, difficulty to print etc., it is commonly employed to attach an interlocking device prepared beforehand on to a film substrate by heat sealing.
As the materials for interlocking devices, polyethylene type resin and polypropylene type resin are widely adopted. In the case the heat-sealing part of interlocking device is polypropylene type resin, the following laminated film are used as a bag's body film substrate for heat sealing: OPP/CPP (oriented polypropylene/non-oriented polypropylene), ONy/CPP (oriented nylon/non-oriented polypropylene), PET/CPP (oriented polyester/non-oriented polypropylene), ONy/Al/CPP (oriented nylon/aluminum foil/non-oriented polypropylene), and PET/Al/CPP (oriented polyester/aluminum foil/non-oriented polypropylene) etc. In recent years, the improvement of film functionalities makes it easier to blocking microorganism completely by film packaging, and it is becoming more popular to transforming bottles and cans to retort pouches. With such transformation, an interlocking device is become required high heat resistance in addition to its original functionality of easy-reclosability.
The interlocking device made of polypropylene type resin used so far has good low temperature heat sealability to a film substrate, however, it has problems such that it is deformed, shrunk or melt bonded by the heat applied during the retort sterilization and loses not only its sealing tightness, but also its reclosability. Furthermore, those polypropylene type interlocking devices used so far usually have a single layer structure, not a multi layer structure so that the polypropylene type resin having high melting temperature shall be used to make the interlocking device which does not deform, shrink and melt bond by the heat applied during retort sterilization. This has a disadvantage that the heat sealing temperature must be increased. As therefore, there is no interlocking device having single layered structure with both low temperature heat sealability and heat stability sufficient to stand under retort sterilization.